1. Field of the Invention
The invention relates to curable mixtures, containing compounds with α,β-unsaturated carbonyl groups and compounds containing CH-acidic methylene groups and trialkylphosphine catalysts.
2. Description of Related Art
Curable mixtures containing compounds with α,β-unsaturated carbonyl groups such as acrylates methacrylates, fumarates, maleates or itaconates and activated carbonyl compounds, containing C—H acidic methylene groups as for example acetoacetates, malonates, β-diketones and β-cyanoacetates are useful as coating materials for paper, wood, metal, plastics, as binders in printing inks, adhesives and for the encapsulation of semiconductor materials. The curing of these mixtures, hereinafter also termed as crosslinking or hardening, is achieved by reaction of the activated double bonds of the α,β-unsaturated carbonyl groups with the C—H acidic methylene groups of the activated carbonyl compounds in presence of a strong basic catalyst.
Examples of such reactions are described in the German patent specification 835809. Curable mixtures containing acrylates and acetoacetates are described in the U.S. Pat. No. 4,408,018. Curable mixtures of malonates and acrylates are disclosed in the U.S. Pat. No. 4,602,061. Such curable mixtures are particularly suitable for hardening of coating materials, as the crosslinking reaction the so called “Michael addition” (Organikum, VEB Deutscher Verlag der Wissenschaften, 16th edition, Berlin 1986, page 509-510) is fast also at low temperatures and no volatile organic splitting products are formed, which may contaminate the environment. However, the reaction requires a strong basic catalyst (pk>12), preferred catalysts are for example amidines such as 1,8-diazabicyclo(5.4.0)undec-7-ene, 1,5-diazabicyclo(4.3.0)non-5-ene or tetramethylguanidine. The disadvantage of this process is that the strong basic catalyst remains in the cured coating, paint or ink and may cause problems. It is a matter of common knowledge, that for example strong amines may cause yellowing. Another drawback of coatings, derived form acetoacetates, acrylates and strong amines such as amidines, is their hydrolysis sensitivity. This is also described in literature (Journal of Coatings Technology, Vol. 61, No. 770, March 1989, page 89). The authors attribute this to the high basisity of the amines, which remain in the cured product and promote ester hydrolysis in the presence of humidity. Amines having lower pk of about 10 are not suitable for the Michael addition of compounds containing α,β-unsaturated carbonyl groups and compounds containing CH-acidic methylene groups. Inorganic bases such as potassium hydroxide or tetramethylammonium hydroxide, which may be used as well, should even increase the hydrolysis sensitivity.